Add agentgateway to CI & rename dataplane flag

[Keith Mattix II (keithmattix)]

Run agentgateway data plane tests as a part of substrate CI (non-blocking to start so we can confirm it's not flaky). Also, change the --atenet-router flag to --dataplane to make it clearer that the flag controls ingress and egress.

I've run the e2es locally across gVisor and microVM plus the MITM variants for both. The only skip we do for agentgateway is TestIngressProtocolDowngrade because 1. the behavior its testing only exists on the non-CONNECT atunnel ingress path and agentgateway only sends CONNECT to atunnel and 2. I'm not sure that we want this to be a part of the contract that substrate is bound by (e.g. do we really want to commit to atunnel always parsing HTTP?).

My goal with getting both dataplanes into CI is to start taking steps to codify the proxy (router + egress PEP) contract for substrate. The telemetry they emit, atunnel expectations, etc. are all important contracts to explicitly call out so that they don't become too coupled to a single dataplane implementation.

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR